Kinetic and mechanistic investigations of enzyme-catalyzed ring-opening polymerizations of lactones.

摘要

Studies were undertaken to gain mechanistic information on enzyme-catalyzed polymerizations by analyzing the propagation kinetics of different lipase-catalyzed ring-opening polymerization of lactones. Extensive work was conducted on two monomer:catalyst systems: (i) {dollar}varepsilon{dollar}-Caprolactone ({dollar}varepsilon{dollar}-CL) catalyzed by porcine pancreatic lipase and (ii) {dollar}omega{dollar}-Pentadecalactone (PDL) using the preferred lipase from Pseudomonas sp., in an immobilized form. The polymerizations of {dollar}varepsilon{dollar}-CL were carried out at low water levels and supplemented with either butanol or butylamine. Rates of monomer conversion, product molecular weight, total chain number, and chain end structure were determined by {dollar}sp1{dollar}H NMR. In the presence of water alone, a maximum M{dollar}rmsb{lcub}n{rcub}{dollar} of 7600 g/mole was obtained at 85% conversion which decreased to 4200 g/mole as the reaction continued to 98% conversion. Reactions with butanol and butylamine at 100% conversion gave polymers with M{dollar}sb{lcub}n{rcub}{dollar} values of 1900 and 1200 g/mole, respectively. For these three polymerizations, the total number of polymer-chains increased with conversion due to a simultaneous increase in carboxylic acid chain ends. Within 4h, butylamine was completely consumed but only 37% of butanol reacted. Reactions with butylamine occurred predominantly by an enzyme-mediated route. In addition, the living or immortal nature of the polymerizations were assessed from plots of Ln{dollar}rm{lcub}{9B}M!sb0{9B}M!sb{lcub}t{rcub}{rcub}{dollar} versus time and M{dollar}rmsb{lcub}n{rcub}{dollar} versus conversion. These results obtained from low M{dollar}rmsb{lcub}n{rcub}{dollar} products indicate that termination and chain transfer did not occur. An expression for the rate of propagation, R{dollar}sb{lcub}p{rcub}{dollar}, was also derived from the experimental data which is consistent with that derived from the proposed enzyme-catalyzed polymerization mechanism (EAM). It was concluded that this is a chain-reacted mechanism with a rate of initiation that is faster than propagation when initiating the ring-opening of {dollar}varepsilon{dollar}-CL with butylamine. The absence of termination in conjunction with the relationship between molecular weight and the total concentration of multiple initiators suggest that {dollar}varepsilon{dollar}-CL polymerization by PPL catalysis shares many features of immortal polymerizations.; The propagation kinetics ascertained from the ring-opening polymerization of PDL using the lipase from a Pseudomonas sp. immobilized on Celite 521 was in contrast to that of {dollar}varepsilon{dollar}-CL. Faster rates of monomer conversion and higher degrees of polymerization resulted from PDL reactions carried out in the bulk at 50{dollar}spcirc{dollar}C using water as the nucleopile. The final products isolated at 40% conversion had M{dollar}rmsb{lcub}n{rcub} < 5,000{dollar} g/mole and MWD of 1.1. Characterization of products at low conversion and the kinetic data accumulated in this work is also consistent with an EAM mechanism. Interestingly, this chain-reacted mechanism proceeds with a faster rate of propagation relative to initiation. The living nature of PDL polymerizations deviated from an ideal living system with (i) a slow increase in propagating chains, (ii) M{dollar}rmsb{lcub}n{rcub}{dollar} values that are higher than the theoretical M{dollar}rmsb{lcub}n{rcub}{dollar}, (iii) MWD decreasing with increasing conversions, (iv) a low initiator efficiency and (v) a R{dollar}sb{lcub}p{rcub}{dollar} that varies with the total number of polymer chains. Comparative studies were also made between the experimental results obtained herein to that of slow-initiating, living polymerization and a living polymerization propagating by slow exchanges between dormant and active chain ends. In conclusion, an empirical active site and substrate